There are three main categories of airdrop today. For highly critical package delivery, the JPADS guided parafoil system is an option that can be employed. Once released from an aircraft these parafoils continuously compensate for the effects of release point error and wind variations using a closed loop feedback control system that translates position errors into actuator commands that continuously vary the aerodynamic characteristics of the parafoil in order to steer the package towards the desired point of impact. While quite effective, these systems are expensive to procure (approximate cost is $36K per unit). Furthermore, because of their high cost, ground personnel are under pressure to retrieve the units for reuse. At the other end of the spectrum, traditional ballistic airdrop is used to deliver the bulk of airdropped packages at a very low cost. Low altitude release is often used when attempting to deliver packages with a high degree of precision and accuracy. Low altitude drops can expose aircraft and crew to risks from enemy fire and terrain; therefore, high-altitude drops are preferred from the standpoint of air safety. High-altitude low-opening drops typically result in lower precision and accuracy than the alternatives described above because uncertainties, drop errors, and winds act upon the dynamics of the system over a longer period of time without any means of correcting for their effects after release from the aircraft. The accuracy of HALO drops is governed by the accuracy of the computed air release point (CARP), how closely the pilot and crew are able to release the packages to the CARP, the accuracy of the dynamic models of the chutes and packages, and the accuracy of the wind field estimate. Most airdrops involve the release of multiple packages. Because of differences in the exit time of each package from the aircraft, and the high speed of the aircraft, the packages typically are dispersed along the ground track of the aircraft prior to stabilizing under the drogue chutes. Furthermore, standard practice is to set the drogue to main chute transition altitudes of all of the packages in a multi-package drop equal to one another. In the presence of no wind and no uncertainties, the ground impact pattern of the packages will closely resemble the dispersion pattern of the packages that existed when the packages first stabilized under their drogue chutes. Thus, the current practice for ballistic HALO drops is completely open loop with no means of reducing unavoidable impact dispersion resulting from differences in release times, and includes no means for compensating for the effects of drop errors, uncertainties, and disturbances.
Summary of advantages and disadvantages of guided parafoils:
Advantages: accurate.
Disadvantages: expensive, high-complexity, pressure to recover hardware exposes ground personnel to danger
Summary of advantages and disadvantages of conventional ballistic airdrop:
Advantages: Low cost, low complexity, well-established procedures
Disadvantages: low accuracy, improving accuracy exposes aircraft and crew to danger